The present invention relates to homopolar machines and more particularly to solid sliding current collectors or brushes for use with homopolar generators and motors.
Homopolar machines have been successfully designed for providing peak electrical current discharges lasting several seconds and producing a peak current level in excess of a million amperes direct current. Such machines generally include a cylindrical rotor of either a drum or disc configuration, mounted on a frame, to be rotated about an axis through the center of the cylinder. A field coil encircling the rotor and connected to an external current supply provides an applied field excitation passing through the rotor. The applied field excitation is usually confined and directed by a ferromagnetic yoke surrounding the field coil and all, or a portion of, the rotor.
A typical homopolar machine discharge is described as follows. The energy storing rotor or flywheel is accelerated to high speed, then the external magnetic field is applied via the excitation coil to create a voltage across the spinning rotor. When full field is reached, an external load is connected across the rotor terminals through sliding contacts or brushes. As current begins to flow through the rotor and load, Lorenz forces decelerate the rotor quickly, accomplishing the conversion of the stored kinetic energy to a single electrical current pulse. Pulse lengths depend upon the characteristics of the external load and are typically from 0.5 to 3.0 seconds in duration.
Although homopolar machines are typically operated in the pulsed mode as described, similar machines may also find application as continuous duty, low voltage, high current generators, or as low speed, high torque motors.
When the homopolar machine rotor is spinning, the free electrons within the rotor experience an electromotive force resulting from their interaction with the applied field excitation. In prior art machines, brushes positioned inside the field coil, or between two halves of the field coil, are then lowered onto a radially outward (circumferential) surface of the spinning rotor to allow a current to flow under the influence of such electromotive force to an external circuit, and then back into the rotor through return conductors and additional brushes at a different location. To complete the electrical circuit, at least two sets of such brushes are required. During the discharge, the interaction of the discharge current and the applied field excitation creates a force which decelerates the rotor until its rotation stops and the discharge ends.
It has been found that extremely high current of short duration pulses may be obtained after using a relatively low power conventional prime mover or a conventional low voltage, low amperage power source to store inertial energy in the rotor by gradually accelerating the rotor up to the desired rotational speed.
In known homopolar generators, the brush mechanisms are subjected to extraordinarily difficult duty. In fact, collection of current by the brushes at the high peripheral speeds attained by the rotor represents the single most demanding task of any pulsed homopolar generator component. The desired pulse current magnitude and amount of stored or pulse energy both affect the brush collector area required to transfer the discharge current from the rotor. A fraction of the stored energy is necessarily lost at the brush/rotor interface in the form of heat, which can be reduced by increasing the collector area.
Performance of homopolar machine brushes is also influenced by two external factors: (1) electrical load characteristics, and (2) the method by which the circuit is closed to initiate a discharge. Resistive loads are distinguished by very fast rise times, as peak current is often reached in less than 30 milliseconds (ms). Time available for the brush to become seated before carrying full current is therefore extremely limited. Inductive loads, on the other hand, slow the rise time considerably, but also extend the current pulse duration as the rotor energy is transferred to the inductor. This longer current pulse can significantly increase brush wear rates because the interface flash temperature, the peak temperature reached at the sliding brush/rotor interface, is maintained for extended periods, thus softening the brush material.
Homopolar machines are typically switched by one of two methods. An external closing switch may be used, or the load may be connected directly to the brushes with switching performed by actuating the brushes into contact with the rotor. If the latter method is chosen, an arc may be drawn as contact is made, causing pitting of the brush surface and corresponding increased wear, thus reducing useful brush life.
The foregoing is provided by way of background only. The general state of the art relating to pulsed power homopolar machines and generators is described more fully in the following publications, which are incorporated herein by reference: U.S. Pat. No. 4,459,504 to W. F. Weldon et al., issued Jul. 10, 1984; U.S. Pat. No. 4,544,874 to W. F. Weldon et al., issued Oct. 1, 1985; U.S. Pat. No. 4,816,709 to Weldon, issued Mar. 28, 1989; W. F. Weldon et al., "The Design, Fabrication, and Testing of a Five Megajoule Homopolar Motor-Generator," presented at the International Conference on Energy Storage, Compression and Switching in Torino, Italy (November 1974); and J. H. Gully, "Compact Homopolar Generator," IEEE Transactions on Magnetics, vol. MAG-18, No. 1 (January 1982). In addition, U.S. Pat. No. 4,816,709 to W. F. Weldon, issued Mar. 18, 1989, describes an energy density homopolar generator as well as the general state of the art of homopolar generators.
The successful development of solid sliding current collectors, or brushes, has advanced significantly over the past two decades (reference U.S. Pat. Nos. 4,459,504 and 4,562,368), driven significantly by the requirements of pulsed homopolar machines. The complex brush mechanisms that have resulted represent a significant fraction of the cost of such machines, both in terms of initial manufacturing cost and continuing maintenance and replacement cost. These costs are primarily a function of the multiplicity of parts that result from the following requirements. Specifically, the brushes must:
(1) be retractable from the rotor surface to reduce wear and losses when not generating; PA1 (2) be capable of rapid actuation to enable use as a closing switch; PA1 (3) have a zero backlash mechanism to provide consistent repeatable alignment of the brush wear surface with the rotor or slip ring, as the brush wear rate has been found to be extremely sensitive to proper brush alignment; PA1 (4) have low inertia to follow rotor runout; PA1 (5) be current compensated to increase brush down force with increasing current levels to prevent arcing; and PA1 (6) provide sufficient brush length for adequate wear allowance.
Meeting these diverse requirements has resulted in a multiplicity of parts to be manufactured and assembled, or to be disassembled and reassembled in the course of machine maintenance.
The physical constraints of known brush mechanisms reduce their efficiency. For example, the space required for the known brush mechanisms results in only a fraction of the rotor periphery being contacted by brushes. Further the curvature of the brush surface of known brush mechanisms must precisely match that of the rotor under varying conditions of wear, temperature and alignment. Even the smallest mismatch in curvature will result in substantial reduction in effective contact area, increased contact impedance, increased brush wear, and therefore decreased machine performance.